Editing Varvara

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'''Varvara''' is a fantasy computer system that uses runs a stack-based virtual machine called '''Uxn'''. Its main programming language is a Forth-like assembly language called '''Uxntal'''.

== Uxn ==

=== Memory ===
The Uxn virtual CPU can address 64kb of memory; programs can move stacks into addressable range to allow reading and writing directly into stack memory. In this way, peripherals such as a controller, mouse, and keyboard can be addressed.

{| class="wikitable" 
|- style="vertical-align:top;"
! Memory
| RAM
| Data
| 64kb
|- style="vertical-align:top;"
!rowspan="6"|Stacks
|rowspan="3"|Working Stack
| Data
| 254 bytes
|-
| Error
| 1 byte
|-
| Pointer
| 1 byte
|- style="vertical-align:top;"
|rowspan="3"|Return Stack
| Data
| 254 bytes
|-
| Error
| 1 byte
|-
| Pointer
| 1 byte
|-
! I/O
| Devices
| Data
| 256 bytes
|}
Uxn is a single-page virtual machine with 32 opcodes. This system has no registers, and instead uses a 127-level stack.

== Devices ==

Devices are external systems connected to the Uxn CPU, such as the screen, the mouse and the keyboard. Each device has 16 bytes, also called ports, of I/O memory. Vectors are ports holding an address in memory to evaluate when a device event is triggered, such as when the mouse is moved, or a key is pressed.

* 00 - System device
* 10 - Console device
* 20 - Screen device
* 30, 40, 50, 60 - Audio device
* 70 - MIDI device
* 80 - Controller
* 90 - Mouse
* a0, b0 - File
* c0 - Datetime
* d0 - Unused
* e0, f0 - Reserved

The two reserved devices can be used for implementation specific features that do not need to be part of the specs, or other Uxn/Varvara instances.

== Uxntal ==

According to the implentation spec, "Tal is the programming language for the Uxn virtual machine." Tal, commonly referred to as '''Uxntal''', is a unique flavor of assembly language. Uxntal source files are assembled from human-readable source files into Uxn-compatible ROM files with the .tal extension.

The language itself works similarly to Forth, another stack-based language.

=== Opcodes ===

Uxn can perform 32 different operations, and each operation has 3 possible modes. The items in the following table include the Program Counter(PC), Memory(M), Devices(D) and Return Stack(rs).

{| class="wikitable" 
|-
!colspan="2"|Stack
!colspan="2"|Memory
|-
| '''BRK/LIT''' - Literal
| a b c M[PC+1]
| '''LDZ''' - Load Zeropage
| a b M[c8]
|-
| '''INC''' - Increment
| a b c+1
| '''STZ''' - Save Zeropage
| a {M[c8]=b}
|-
| '''POP''' - Pop
| a b
| '''LDR''' - Load Rel
| a b M[PC+c8]
|-
| '''NIP''' - Nip
| a c
| '''STR''' - Save Rel
| a {M[PC+c8]=b}
|-
| '''SWP''' - Swap
| a c b
| '''LDA''' - Load Abs
| a b M[c16]
|-
| '''ROT''' - Rotate
| c b a
| '''STA''' - Save Abs
| a {M[c16]=b}
|-
| '''DUP''' - Duplicate
| a b c c
| '''DEI''' - Device In
| a b D[c8]
|-
| '''OVR''' - Over
| a b c b
| '''DEO''' - Device Out
| a {D[c8]=b}
|-
!colspan="2"|Logic
!colspan="2"|Arithmetic
|-
| '''EQU''' - Equal
| a b?c
| '''ADD''' - Add
| a b+c
|-
| '''NEQ''' - Not Equal
| a b!c
| '''SUB''' - Subtract
| a b-c
|-
| '''GTH''' - Greater
| a b>c
| '''MUL''' - Multiply
| a b*c
|-
| '''LTH''' - Lesser
| a b<c
| '''DIV''' - Divide
| a b/c<br />
|-
| '''JMP''' - Jump<br />
| a b {PC+=c}
| '''AND''' - And
| a b&c
|-
| '''JCN''' - JumpCond
| a {(b8)PC+=c}
| '''ORA''' - Or
| <nowiki>a b|c</nowiki>
|-
| '''JSR''' - JumpStash
| a b {rs.PC PC+=c}
| '''EOR''' - ExclusiveOr
| a b^c
|-
| '''STH''' - Stash
| a b {rs.c}
| '''SFT''' - Shift
| a b>>c8l<<c8h
|}

[[Category: Fantasy computers]]